1. Field of the Invention
The present invention relates to the field of solid-state image sensing devices and, more specifically, to CMOS based active pixel sensors.
2. Description of the Prior Art
CMOS active pixel image sensors have been rapidly developed to supplement and replace charge-coupled devices in many applications. CMOS image sensors have advantages in low-power operation as well as the higher integration capabilities for on-chip signal processing and control functions that are available with CMOS technology. Therefore, the technology is suitable for many consumer applications, including low-cost portable imaging devices. CMOS based sensors allow for the incorporation of numerous functions onto a single chip. Various prior art references have disclosed Active Pixel Sensors based on CMOS that employ sense node connected to photodetectors through transfer gates. Typically a reset gate is provided that can be used for shuttering purposes. However, in the application areas of industrial cameras and high-frame rate recording, CMOS sensors have been handicapped by embodiments which employ row-by-row exposure controls which allows a slit shuttering mode that is not capable of simultaneous exposure of the entire CMOS pixel array. An additional shutter gate is added to provide direct reset of the photodetector independent of the transfer gate. However, the addition of the gate may cause increased fixed pattern noise due to non-optimal removal rates for charges stored within the photodetectors in the prior art devices.
From the foregoing discussion, it should be readily apparent that there remains a need within the art for a CMOS based Active Pixel Sensor design that provides for independent electronic shuttering while providing optimal removal rates for charges stored within the photodetectors.